1. Field of the Invention
The present invention relates to an improvement in a yoke of stepping motor and method for manufacturing the same.
2. Description of the Prior Art
In recent years, a permanent-magnet type stepping motor (hereinafter abbreviated as PM type stepping motor) has been advanced to be light-weighted, thinned, shortened and small-sized. Furthermore, the PM type stepping motor is required to be manufactured in a low cost and to have high performance.
A conventional PM type stepping motor 10 is described referring to FIGS. 1 and 2. FIG. 1 is a perspective view showing conventional yokes 3 and 3 which are used in the conventional stepping motor 10. FIG. 2 is a cross-sectional side view showing the constitution of the conventional and typical stepping motor 10.
In FIG. 1, which is an exploded perspective view of an upper stator 11a, an upper yoke 31 and a lower yoke 32, which are made of a magnetic material such as low carbon steel sheet, respectively have plural upward pole teeth 31a, . . . and plural downward pole teeth 32a, . . . on their inner periphery parts. A coil is wound on a bobbin 2 of an insulation material. The upper yoke 31 and the lower yoke 32 are fitted to the upper and the lower faces 2a and 2b of the bobbin 2, respectively. The pole teeth 31a, . . . and 32a, . . . are fit in an inner hole 2c of the bobbin 2 in a manner that the pole teeth 31a, . . . of the upper yoke 31 and the pole teeth 32a, . . . of the lower yoke 32 make alternate row of circularly disposed magnetic pole pieces. Thus, an assembly of the upper yoke 31, the coil 1 on the bobbin 2 and the lower yoke 32 constitutes the upper stator part 11a.
As shown in FIG. 2, the stator 11 comprises the upper stator part 11a and a lower stator part 11b. The lower part 11b is configurated quite equivalent to the upper part 11a. The upper part 11a and the lower part 11b of the stator 11 are tightly piled up. The coils 1a and 1b of the upper and lower parts 11a and 11b of the stator 11 are, for example, alternately magnetized by a known stepping-motor driving circuitry. A cup-shaped frame 4 is made of a magnetic material e.g. dead-soft steel sheet. Outer peripheries of the stator 11, namely outer periphery edges of the yokes 31, 31 and 32, 32 contact the inner vertical face 4b of the frame 4, to form magnetic circuits. The frame 4 has a bearing 6 at its top center part for rotatably bearing a shaft of a rotor 5. The rotor 5 comprises a cylindrical core 5b made of non-magnetic die-cast or industrial resin and a plurality of vertical rod-shaped permanent magnets 5c, . . . provided circularly on cylindrical outer face of the core 5b. The rotor 5 is rotated by magnetic attraction and magnetic repulsion between the pole teeth 31a, . . . and 32a, . . . and the permanent magnets 5c, . . . . A bracket 8 having a bearing 7 for rotatably bearing lower part of the shaft 5a of the rotor 5 is fit in an opening 4a of the frame 4.
In the conventional PM type stepping motor 10 constituted as mentioned above, the yokes 3, . . . are made of galvanized low carbon steel sheet (galvanized steel sheet) which has a good workability. And the yokes 3, . . . are formed by press working. Such a galvanized low carbon steel, however, has low permeability as a magnetic material after being press worked. Therefore, the PM type stepping motor using the yokes 3, . . . made of such a galvanized low carbon steel sheet after press working can not have high performance. Accordingly, for improving the performance of the PM type stepping motor 10, the yokes 3, . . . made of the galvanized low carbon steel sheet must be annealed for increasing the permeability thereof after forming to the shape shown in FIG. 1 by the press working. Such annealing of the yokes 3, . . . is executed in an inactive gas atmosphere at a temperature of about 800.degree. C. for relieving internal stress caused by the press working so as to increase the permeability of the yokes 3, . . . .
In the above-mentioned case, zinc on the galvanized steel, however, is melted by such a high temperature during the annealing. Therefore, there is a problem that the annealed yoke 3 lose rust-preventive nature. For solving the problem, a low carbon steel which has not been treated with any surface treatment is used and the above-mentioned annealing is executed after forming the yokes 3, . . . by the press working. In this case, however, there is another problem that a rust-preventive plating process is necessary after the annealing. Furthermore, there is still other problems that the yokes 3, . . . formed by the press working may be deformed during in cargo working for carrying the yokes 3, . . . to and from the plating shop and that the deformed yokes 3, . . . have to be corrected.